Thermosetting resins used in casting or open and closed mold applications are typically cured by a free radical polymerization process. Examples of such thermosetting resin include unsaturated polyester resins, vinyl ester resins and urethane (meth)acrylates. The backbone of these resins either contain ethylenically unsaturated groups such as fumarate or (meth)acrylate and are dissolved in a liquid copolymerizable monomer such as styrene, methyl methacrylate or vinyl toluene. Such resins are liquid under normal conditions, though when treated with a source of free radicals such as an organic peroxide initiator an in the presence of a promoter will rapidly crosslink to form a hard thermoset crosslinked network. Such a process is used in the production of, for example, castings, coatings, adhesives and fiber reinforced articles.
Commercially available promoter systems for ambient cure thermosetting resins include accelerators or promoters used in conjunction with the initiator and include, for example, salts of metals chosen from among lithium, calcium, copper, vanadium, zirconium, titanium, nickel, iron, sodium, potassium, magnesium, manganese, barium and cobalt, in combination with one or more compounds of alkyl organic acids, halides, nitrates to form a coordination compound. The choice of the metal ion of metal salts depends upon several parameters, such as activity at ambient temperatures, possible coloring effects, toxicity, stability in the thermoset product, price, and the like. It should be taken into account that the activity of the metal ion also depends upon the kind of coordinating groups. Because of their good performance at ambient temperature, cobalt-containing accelerators are the most widely used copromoters. However, a disadvantage of cobalt is that cobalt carboxylates are suspect to high toxicity (carcinogenicity). Hence, there is an increasing demand in the thermosetting resin industry for promoters that can provide an appropriate curing without compromising performance of the resulting products.
Much attention has recently been given to thermosetting systems that can be cured via free radical polymerization together with a variety of accelerators. In particular, there is interest in accelerators that are free of any cobalt salts such as cobalt carboxylates. Such cobalt carboxylates are believed to have a high toxicity (carcinogenicity). Various patents describe promoter systems that do not include cobalt salts and that are able to cure thermosetting resins and are described for example in U.S. Pat. No. 8,039,559, WO 2005/047379 A1 and WO 2006/131295 A1, the disclosures of which are incorporated herein by reference in their entirety.
WO 2011/083309 A1 and WO 2011/124282 A1 describe the preparation of accelerators based on iron/manganese complexes of tridentate, tetradentate, pentadentate, or hexadentate nitrogen donor ligands. These publications describe metal iron and manganese metal complexes able to cure unsaturated polyesters and vinyl esters and do not include any cobalt salts. The nitrogen ligands are made from structures which present some difficulty on the preparation of their chemical structures. Multiple steps are required in there preparation which make the products more expensive due to their fabrication problems.
It is also known to use various copper complexes. For example, U.S. Pat. No. 8,722,770 describes curing of unsaturated polyesters and vinyl ester resins using accelerators comprising of copper compounds and acetoacetamide compounds in combination with a potassium carboxylate under the influence of a peroxide. Additionally, an alkaline metal salt such as a potassium carboxylate is also included.
U.S. Pat. No. 8,008,380 discloses complexing agents such as triethanolamine and bipyridine and salts of metal selected from magnesium, lithium, manganese, copper, iron and potassium with optionally a solvent. Additionally, the accelerator solution may include stabilizers such as diethyl acetoacetamide, methyl acetoacetate, trimethyl phosphate, dibutyl phosphate and ethyl acetonate.
U.S. Pat. No. 8,039,559 describes curing of unsaturated polyesters and vinyl ester resins using accelerators that comprise a transition metal compound selected from copper, iron, manganese or titanium and a potassium compound in combination with a 1,3-dioxo compound such as acetyl acetone.
U.S. Publication No. 2013/00338317 relates to a pre-accelerated unsaturated polyester or vinyl ester resin composition comprising a soluble copper compound and a heterocyclic aromatic amine such as imidazole and which resin composition is essentially free of cobalt. The copper and the heterocyclic amines are added in a sufficient amount to accelerate a peroxide curing. Additionally, 1,3-dioxo compounds are also incorporated into the accelerator compositions.
U.S. Pat. Nos. 2,593,787; 2,740,765; 3,437,715 and 3,840,618 describe unsaturated polyester resins containing quaternary ammonium and copper salts which provide extended periods of storage for uncatalized conditions. Upon addition of a polymerization catalyst and high temperatures, short curing times are provided.
U.S. Pat. No. 3,091,936 describes thermosetting resinous compositions capable of rapidly curing using cobalt salts together with N,N-dialkylaryl tertiary amines. The patent also describes stabilization of the resin using copper and quaternary ammonium salts.
There remains the need to have metal complexes that do not include cobalt to cure thermosetting resin systems via free radical polymerization at room temperature or at moderate temperatures, with excellent processability, without compromising their mechanical properties. It also would be desirable to provide a thermosetting resin that has a stable gel time over a specified life time with minimum variation. In addition, it would be advantageous to have an efficient process that would yield products free of toxic or harmful components in the resulting thermosetting materials.